The present invention relates to a photopolymerizable composition that contains a polymeric binder; an ethylenically unsaturated, polymerizable compound; and a photoinitiator.
Compositions of this kind are used for producing printing plates, relief images and photoresists and are known, for example, from German patents No. 1,171,267, No. 2,027,467 and No. 2,039,861, and from German Offenlegungsschriften No. 15 22 575, No. 20 64 079 and No. 20 64 080 (corresponding to U.S. Pat. No. No. 3,804,631). They are employed, in particular, as recording materials for the photomechanical production of lithographic printing plates and for photoresist processes.
In the photopolymerizable layers of these compositions or reproduction materials, respectively, dyes or pigments are usually included to improve the visibility of the relief image following development. The colored additives should be lightfast by nature, since the stencil obtained after development, which consists of the light-hardened layer constituents, must exhibit a maximum of contrast.
As a further component, these compositions contain photoinitiators which absorb the light of metal halide lamps (.lambda.&gt;350 nm) used in commercial copying apparatus.
It is known that organic halogen compounds absorb light and split off radicals capable of initiating a photopolymerization reaction. As a group, however, these organic halogen compounds absorb light only in the short-wave UV region (.lambda.&lt;350 nm).
Attempts have been made to shift the absorption of compounds of this kind into the longer-wavelength region by incorporating chromophoric radicals in heterocycles containing trichloromethyl groups. Absorption bands between 350 and 450 nm are described for triazine derivatives in German Offenlegungsschriften No. 22 43 621 and No. 27 18 259; for oxadiazole derivatives in German Offenlegungsschriften No. 28 51 472 and No. 29 49 396, for oxazoles in German Offenlegungsschriften No. 30 21 590 and No. 30 21 599; and for thiazoles in European Patent Applications No. 0 135 348 and No. 0 135 863.
Moreover, it has been indicated in the literature that halogen compounds can be sensitized for photoreaction by adding sensitizers, some of which are moderately active. German Offenlegungsschriften No. 28 51 641 and No. 29 34 758 describe a combination of trihalogenomethyltriazines with 2-benzoylmethylenenaphtho[1,2-d]thiazole, while German Offenlegungsschrift No. 26 10 842 describes a mixture composed of N-phenylacridone and trihalogenomethyl compounds. It is also possible to sensitize .alpha.-halogeno-carboxamides (German Offenlegungsschrift No. 27 18 200) or tribromomethyl-phenylsulfones (German Offenlegungsschrift No. 35 03 113) by means of photoinitiators, such as benzophenone, benzil and Michler's ketone.
The goal of these attempts to shift the absorption of organic halogen compounds has been to increase the light sensitivity of photopolymerizable compositions, by an improved light absorption of the photoinitiator system. At present, fulfillment of this goal is still sought after.